The invention relates to a method of reducing an oxygen-bound metal in a furnace heated with an oxidizing flame. More particularly, a first layer, including a mixture of a metal oxide and a reductant, are placed onto an annular platform within the furnace. The first layer is covered by a reductant second layer to prevent reoxidation of the reduced metal oxide. The reduced metal oxide may be used as feed stock for supplying metal units into a refining furnace for alloying with molten iron.
It is known to reduce metal oxides and metal ores with the reduced product being subsequently used as a feed stock in a refining vessel for making iron or iron alloys. The type of furnaces commonly used for reducing metal ores are vertical shaft, rotary kiln or rotary hearth. U.S. Pat. No. 4,701,214, incorporated herein by reference, discloses using a rotary hearth furnace wherein iron ore, coal and lime are mixed together, pulverized and compacted into pellets. The iron ore pellets are fed two to three pellets deep onto a rotating platform within the furnace. The iron ore pellets are heated to 1000.degree. C. by burners positioned annularly above the rotating platform. A reducing fuel of hydrogen and carbon monoxide is used in the burners. After being reduced, the iron ore pellets then are fed into a smelting furnace to be dissolved into molten iron.
U.S. Pat. No. 4,622,905 discloses using a rotary hearth furnace to metallize iron oxide. Pellets of iron oxide and coal powder are placed onto a rotating platform within the furnace. The pellets are heated by a luminous flame from burners positioned annularly above the rotating platform. The heating flame is formed from the combustion of powdered coal and oxygen.
U.S. Pat. No. 5,166,741 discloses using a rotary hearth furnace to metallize iron oxide steel plant dusts containing heavy metals. Pellets of iron oxide, carbonaceous material, a binder and optionally calcium oxide are formed into pellets. The pellets are placed onto a turntable in the rotary hearth furnace. The pellets are dried at a temperature no higher than 900.degree. C. for up to 15 minutes and then reduced for up to 30 minutes at a temperature of 1150.degree. C.
U.S. Pat. No. 3,443,931 discloses using a rotary hearth furnace to metallize iron oxide. Pellets of iron oxide and coal are laid one or two pellets deep onto a rotating platform within the furnace. The pellets are heated in an atmosphere devoid of free oxygen having a temperature up to about 1425.degree. C. by burners positioned above the rotating platform.
U.S. Pat. No. 4,772,316 discloses using a rotary-type furnace to metallize an iron-containing chromium ore to produce ferrochromium used as a master alloy in the manufacturing of chrome steel. A mixture of chromite ore, coal and a flux is heated in a carbon monoxide-containing atmosphere. The mixture is charged into one end of the furnace, flows through the furnace and is continuously discharged from the other end of the furnace. Burner gases are passed through the discharge end of the furnace and flow in a direction counter to the flow of the chromium ore. Coal is used, not only to reduce the combustion product gases of CO.sub.2 and H.sub.2 O to form carbon monoxide and hydrogen, but also to prevent reoxidation of metallized chromium.
U.S. Pat. No. 4,971,622 relates to reducing and desulfurizing chromite ore. A chromite ore is mixed with a carbonaceous material and heated to 1500.degree. C. in a rotary kiln. At least 90 wt. % of the chromium oxide is reduced to a metallic state, and nearly 100 wt. % of the iron oxide is reduced to a metallic state. This reduction product containing calcium oxide and excessive carbon is fed into an electric arc furnace for desulfurization.
It also is known to produce stainless steel by charging chromite ore and/or nickel ore directly into a refining vessel having a top blowing oxygen lance and bottom tuyeres for blowing stirring gas. U.S. Pat. No. 5,047,082 discloses producing stainless steel in an oxygen converter using a low-sulfur nickel ore instead of ferronickel. Nickel ore is reduced by carbon dissolved in molten iron and char present in the slag. U.S. Pat. No. 5,039,480 discloses producing stainless steel in a converter by sequentially smelting and reducing low-sulfur nickel ore and then chromite ore instead of melting ferronickel and ferrochromium. The ores are reduced by carbon dissolved in the molten iron and char present in the slag.
There are disadvantages associated with processing ores in a shaft furnace or a rotary kiln. In a shaft furnace, the ore burden travels downwardly through the shaft toward hot reducing gases rising counter-current through the bed or column of ore. The ore burden does not mix with itself to any great extent during its decent through the shaft. In a kiln, as the ore moves from one end to the other by gravity, there is considerable mixing due to rotation of the kiln. Both furnaces involve movement of the ore relative to the furnace walls, making it more difficult to know and control the temperature of the ore. A shaft furnace and a rotary kiln tend to have sticking or clustering problems. In the shaft furnace, the ore tends to stick to itself, while in the kiln furnace the ore tends to stick to the inside wall of the drum as well as to itself. And, these problems increase, the higher the operating temperatures. Occasionally during operation, a kiln will develop local hot spots where melting of the ore can occur, resulting in severe agglomeration of the ore, which adversely affects production. Both the shaft and kiln furnaces require lump or hardened agglomerated ore because of burden weight in the shaft and impact in the kiln.
A major disadvantage associated with complete processing of ores in a refining vessel during the manufacture of stainless steel is that the ore usually contains the metal in small amounts and is difficult to melt. Also, generally considerable energy is required to reduce the metal oxide into a metal suitable for alloying. Resort is made to a high degree of post combustion, but this necessitates the addition of solid carbonaceous material to the bath. This carbonaceous material is needed to prevent foaming of the bath slag and to prevent reoxidation of the alloys to the slag. The presence of carbonaceous material in the slag in significant quantities results in an enriched carbon concentration of the bath. This carbon has to be removed during refining. Smelting of ore may be further undesirable, especially in the case of low-grade ores such nickel ore, because as much as 80 per cent of the weight of the ore converts to slag. Nickel ores contain only about 1-3 wt. % Ni.
Nevertheless, there remains a long felt need to provide inexpensive metal units for making alloyed iron or steel such as chromium alloyed steel or stainless steel. There also remains a need for providing metal units from inexpensive metal oxides. Other long felt needs include developing a reliable and consistent process for providing inexpensive Cr and/or Ni units for alloying.